feat: Specific starting apps for the editor only view

[gadenbuie]

The app template we use for https://shinylive.io/r/editor currently opens an app with a decent bit of code and comments (compare it with https://shinylive.io/py/editor).

As a heavy user of shinylive.io/r/editor, this adds a decent bit of friction because I need to delete most of the app before I can start working on my example.

This PR modifies the current example flow to add support for a special "Editor" category that, when present, is the starting code for the {appEngine}/editor/ end point. In {appEngine}/examples/ page, the Editor category is filtered out and won't appear in the sidebar.

[georgestagg]

This looks reasonable to me.

If we wanted to avoid the extra complexity we could also just add your minimal example as the first app, named something like 000-basic. However, this way works OK too.